Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes at least one stimulation lead implanted at the desired stimulation site and an Implantable Pulse Generator (IPG) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via one or more lead extensions. Thus, electrical pulses can be delivered from the neurostimulator to the electrodes carried by the stimulation lead(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. The neurostimulation system may further comprise a handheld Remote Control (RC) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The RC may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
Neurostimulation systems, which may not be limited to SCS used to treat chronic pain, are routinely implanted in patients who are in need of Magnetic Resonance Imaging (MRI). Thus, when designing implantable neurostimulation systems, consideration must be given to the possibility that the patient in which a neurostimulator is implanted may be subjected to electro-magnetic energy from MRI scanners, which may potentially cause damage to patient tissue, malfunction or damage or the neurostimulator, and/or discomfort to the patient.
In particular, in MRI, spatial encoding relies on successively applying magnetic field gradients. The magnetic field strength is a function of position and time with the application of gradient fields throughout the imaging process. Gradient fields typically switch gradient coils (or magnets) ON and OFF thousands of times in the acquisition of a single image in the presence of a large static magnetic field. Present-day MRI scanners can have maximum gradient strengths of 100 mT/m and much faster switching times (slew rates) at or exceeding 200 mT/m/ms, which is capable of generating unintended peripheral nerve stimulation in patients even without the presence of an implantable device. Typical MRI scanners create gradient fields in the range of 1 Hz to 10 KHz, and radio frequency (RF) fields of 64 MHz for a 1.5 Tesla scanner and 128 MHz for a 3 Tesla scanner. Both of these types of applied fields are activated in bursts, which have comparable frequencies to stimulation therapy frequencies.
Because the stimulation leads can act as antennas that collect RF energy, the strength of the RF field generated by a conventional MRI scanner may be high enough to induce voltages on to the stimulation lead(s), which in turn, are seen by the IPG electronics, where it can affect the behavior of the IPG and even result in permanent damage. The RF energy induced in the electrodes may not be distributed homogenously, creating certain areas of higher energy concentration. Even if the total RF energy induced on the stimulation leads could be tolerated by the IPG, undesirable high energy pulses or resulting hot spots may impact IPG performance.
The strength of the gradient magnetic field generated by a conventional MRI scanner can also induce voltage on the stimulation leads, which if higher than the voltage supply rails of the IPG electronics, could cause unwanted stimulation to the patient due to the similar frequency band, between the MRI-generated gradient field and intended stimulation energy frequencies for therapy, as well as potentially damaging the electronics within the IPG. In particular, the gradient magnetic field may induce electrical energy within the wires of the stimulation lead(s), which may be conveyed into the circuitry of the IPG and then out to the electrodes of the stimulation leads via the passive charge recovery switches. For example, in a conventional neurostimulation system, an induced voltage at the connector of the IPG that is higher than IPG battery voltage (typically ˜3-5V), may induce such unwanted stimulation currents.
While IPGs can be programmed to switch to a dedicated “MRI mode” that prevents, or at least minimizes, the potentially harmful effects caused by the combination of static, gradient, and RF electromagnetic fields generated by conventional MRIs, known implementations require the neurostimulation system to switch to the dedicated MRI mode prior to or during exposure from the MRI scanner. Therefore, there is a chance that the IPG may not be in the appropriate mode if the IPG resets, if the IPG experiences a failure, if there is failure to detect the occurrence of an MRI, or if there is failure to instruct the IPG to be placed in the MRI mode.
There, thus, remains a need to ensure that an IPG is in an appropriate mode during an MRI.